PROJECT SUMMARY The onset of walking and growth in social communication and language are likely to be intimately connected. Advances in walking facilitate infants? exploration of the environment and lead to alterations in caregiver input; and these changes in turn provide expanded opportunities for the development of social communication and language. Relative to crawlers, walkers have greater access to distal objects, explore the environment more efficiently, move more with objects in hand, initiate social interaction more easily, and carry objects to caregivers in communicative bids creating moments of shared attention during which caregiver language input is likely to be maximally effective. Delays in the emergence and development of walking may therefore reduce opportunities for exploration and social interaction and diminish rich caregiver input beneficial for the growth of social communication and language. The purpose of this research is to conduct a detailed examination of this unstudied developmental pathway by providing prospective, longitudinal data on the emergence and development of walking, changes in locomotor exploration, advances in infant communication, alterations in caregiver communicative input, and later developments in language from 6 to 36 months of age. The study design involves intensive longitudinal observations that combine standard gait data with a rich array of infant motor, exploration, and communicative behaviors and caregiver input to be coded from video. This will permit close tracking of advances in walking, locomotor exploration, and infant and caregiver communication and evaluation of their relationship to later child language outcomes. By comparing infants known to be at heightened risk (HR) for motor and communicative delays (viz., infants with an older sibling diagnosed with Autism Spectrum Disorder) to infants with no known risk (low-risk infants, LR), this research will yield a wealth of data not only on fundamental developmental processes but on the effects of developmental delays during a critical period in development. It will also yield a deeper understanding of the ways in which infant exploration and communication and caregiver input change when infants begin to walk, and thus it will lay the groundwork for the development of novel interventions that can be delivered early with the goal of enhancing social communication and language outcomes in infants with early motor delays with a broad range of etiologies. !